Usually, during tapping, a core hole is produced in the workpiece in a first process step by means of a drill bit. Subsequently, in a second process step, an internal thread is cut in the core hole by means of a separate tapping drill bit. Depending on its size, the tapping drill bit has two or a plurality of cutting lips. The cutting lips have teeth, each of which removes a chip of material from the inner wall of the core hole and, if need be, deforms the material plastically to a small extent. The teeth of the tapping drill bit are different in form; for example, they are flattened to different extents. In this way, when cutting, each tooth entrains with it roughly equally sized material chips.
In distinction to the above tapping process, the generic method occurs using a percussion tapping tool bit, in which the drilling of the core hole and the cutting of the internal thread are carried out in a joint tool bit stroke. At its drill bit tip, the percussion tapping tool bit has a main cutting lip and a thread profile, which trails in a tapping direction and has at least one thread cutting tooth. In the method, there occurs a tapping stroke and subsequently a reversing stroke in the opposite direction. In the tapping stroke, on the one hand, the main cutting lip produces the core hole and, on the other hand, the thread profile forms the internal thread at the inner wall of the core hole until a useable intended thread depth is reached. The tapping stroke is carried out with a tapping feed and at a tapping speed, synchronized therewith, of the tapping tool bit. In a following reversing stroke in the opposite direction, the tapping tool bit is guided out of the threaded bore in a reversing direction, specifically with an opposite reversing feed and at a reversing speed synchronized therewith. In this way, it is ensured that the thread profile of the tapping tool bit is moved in the thread pitch of the internal thread in a stress-free manner.
In the above method, at the end of the tapping stroke, the tapping process is slowed; that is, the tapping feed and the tapping speed synchronized therewith are reduced to 0. However, in the prior art, this retardation of the thread cutting process to a tapping speed of zero has led to an excessively large cutting stress on the thread profile, which can lead to cutting teeth being broken out or the tool bit being broken.
Known from DE 38 80 394 T2 is a combined tool bit for drilling a hole and for cutting a thread. The tapping tool bit is used initially to produce a core hole. Subsequently, the tapping tool bit is moved by its tool bit axis in a circular path around the axis of the bore, specifically with rotation of the tapping tool bit, as a result of which the thread profile forms an internal thread in the core hole. Essentially the same method is known from DE 39 39 795 T2 and from U.S. Pat. No. 5,678,962.
Known from JP 2005 230933 A is a generic method for producing a threaded bore in a workpiece. Tapping tool bits for producing a threaded bore in a workpiece are known from DE 39 21 734 A1, from US 2008/170921 A1 and from DE 100 61 476 A1. Another tapping tool bit is known from DE 18 18 609 U.
An object of the invention is in providing a method for producing a threaded bore in a workpiece as well as a tapping tool bit for carrying out the method, in which the tool bit stress is reduced.
The reversing stroke does not occur immediately after the tapping stroke, but rather follows a groove forming step, in which a pitch-free encircling groove adjoining the internal groove is formed, in which the thread profile of the tapping tool bit can rotate in a stress-free manner. In this way, the tapping speed can be reduced to 0, without an excessively large cutting lip stress leading to the tool bit being broken or the thread profile breaking out.
As mentioned above, the thread profile of the tapping tool bit can rotate in a stress-free manner in the pitch-free encircling groove produced in the groove forming step. In addition, the provision of the encircling groove makes is possible for the tapping tool bit to use a cutting lip to produce an encircling thread countersink in the bore opening of the bore. The encircling thread countersink can thus be produced during the above groove forming step.
In a first embodiment, the tapping tool bit can be utilized as a pre-processing tool bit. In this case, the pre-processed threaded bore must be post-processed in a post-processing step with the help of a finishing tool bit. As a finishing tool bit, it is possible to utilize a thread former, a helical thread former, or an axial thread former. Alternatively to this, in a second embodiment, the thread tapping tool bit itself can be formed as a finishing tool bit. In this case, the above-mentioned additional post-processing step can be dispensed with.
In a technical implementation, the tapping stroke can be lengthened in the tapping direction directly with a groove forming stroke. In this case, the thread tapping tool bit is moved beyond the intended thread depth until an intended bore depth is reached, specifically with a groove forming feed as well as at a groove forming speed that are not synchronized with each other and/or are different from the tapping feed and from the tapping speed.
It is preferred when, at the end of the groove-forming step, the thread profile can rotate completely in the encircling groove of the threaded bore in a stress-free manner. The encircling groove is produced during the groove-forming stroke with the aid of the main cutting lip as well as of the thread cutting tooth (or general thread tooth) of the thread profile at the tapping tool bit.
When the intended bore depth is reached, the groove forming feed is reduced to 0. At the same time, the groove forming speed is also reduced to 0 in order to make possible the reversal in the direction of rotation that is required for the reversing stroke.
At the start of the reversing stroke, the tapping tool bit is controlled in such a way that the thread cutting tooth can be driven in a stress-free manner into the thread pitch outlet, which opens into the encircling groove. Subsequently, the tapping tool bit is guided out of the threaded bore in a direction that is opposite to the tapping direction, specifically with a reversing feed as well as at a reversing speed synchronized therewith, as a result of which the thread cutting tooth can be rotated out of the threaded bore without removal of material.
While the tapping stroke, the groove forming stroke, and the reversing stroke are being carried out, the longitudinal axis of the core hole and the rotational axis of the tapping tool bit preferably remain aligned at all times coaxially to each other.
A tapping tool bit for carrying out such a method can preferably have a clamping shank and a tapping body adjoined to it. Along the longitudinal axis thereof, at least one chip groove can extend to a front-end main cutting lip at the drill bit tip. At the front-end main cutting lip, a chip surface, which bounds the chip groove, and a front-end free surface of the drill bit tip converge. As viewed in the peripheral direction of the tool bit, the chip groove can be bounded by at least one drill bit web. The chip surface of the chip groove can transition, with formation of an auxiliary cutting lip, into a back surface of the drill bit web on the outer peripheral side. At the back surface of the drill bit web on the outer peripheral side, the thread profile can be formed with at least one thread cutting tooth. The tooth height of the thread cutting tooth is dimensioned in the radial direction in such a way that the thread cutting tooth protrudes outward over the main cutting lip in the radial direction by a radial offset. If need be, the thread cutting tooth can flushly extend the main cutting lip outward in the radial direction. Alternatively and/or additionally, as viewed in the axial direction, the thread cutting tooth can be arranged at an axial offset behind the main cutting lip.
In a preferred embodiment variant, the thread tapping tool bit can have three drill bit webs. Each of these drill bit webs is formed with at least one thread cutting tooth. The thread cutting teeth are preferably formed with cutting geometries that are not identical, but are rather different in form. By way of example, in the peripheral direction of the drill bit, a pre-cutting tooth, a middle cutting tooth, and a finishing cutting tooth of different cutting geometry are formed in succession at the drill bit. The cutting teeth are formed at the thread tapping tool bit in an offset manner with respect to each other in the axial direction. The extents of offset thereof are matched to the thread tapping speed and to the tapping feed in such a way that a flawless thread cutting is ensured.
The advantageous embodiments and/or enhancements of the invention explained above and/or presented in the dependent claims—apart from cases of clear dependencies or inconsistent alternatives, for example—can be implemented individually or else in any combination with one another.